This invention relates to a method of finishing a spray-hole or spray-holes bored in the body of a fuel injection nozzle for an internal combustion engine, particularly for a diesel engine, by electrolytic polishing mainly for the purpose of removing burrs existing around the periphery of the inner mouth of each spray-hole.
In a diesel engine, pressurized fuel supplied from a fuel injection pump is ultimately atomized and injected into the combustion chamber by means of a fuel injection nozzle. Naturally the functional characteristics of the injection nozzle greatly influence the engine performance.
In a popular fuel injection nozzle of the type formed with a so-called sac hole in its tip portion, a plurality of spray-holes are bored in the nozzle body wall defining the sac hole each at a predetermined angle with the center axis of the injection nozzle. Usually the spray-holes are bored from the outside by machining such as drilling, and it is inevitable that burrs remain on the inner surface of the nozzle body around the mouth of each spray-hole. Such burrs must be removed because, if left unremoved, the burrs offer increased resistance to the flow of fuel to result in deterioration of the fuel atomizing efficiency and the combustion efficiency. However, it is very difficult to remove the burrs by machining since the burrs exist in the deep of the nozzle body and can hardly be observed by the eye. Usually the burrs are removed by an electrolytic polishing process.
Electrolytic polishing is a process reverse of plating. In an electrolyte liquid such as an aqueous solution of sodium chloride, the metal work to be polished is made the anode in an electric circuit and a separate conductor the cathode. While an electric current is applied through the electrolyte liquid, electrochemical dissolution of the work surfaces proceeds continuously. To prevent accumulation of the dissolved metal ions on the cathode, a relatively high pressure is applied to the electrolyte liquid so that the liquid makes a high velocity flow through the gap between the work and the cathode.
In the conventional electrolytic polishing process for removing the burrs in the fuel injection nozzle body, a tubular electrode (cathode) covered with an insulator is inserted into the nozzle body which is secured to the electropolishing apparatus and made the anode. In an end portion the insulator covering the tubular cathode has a conical outer surface which comes into close contact with the valve seat surface in the nozzle body, and the open end portion of the tubular cathode protrudes from the insulator so as to open into the sac hole at short distances from the spray-holes. In this state electrolytic polishing is performed by injecting the electrolyte liquid into the sac hole through the interior of the tubular cathode. The liquid flows out of the sac hole through the spray-holes, carrying away the metal ions formed by dissolution of the projecting burrs.
In removing the burrs by this method it is inevitable that the metal surface defining the sac hole dissolves to some extent, particularly in a region opposite to the open end of the tubular cathode, with a resultant increase in the volume of the sac hole. Consequently, in a fuel injection nozzle using the nozzle body finished by the electrolytic polishing process an increased dead space exists between the tip of the needle valve in the closing position and the inner surface of the nozzle body. After closing of the needle valve to terminate fuel injection, fuel remaining in the increased dead space dribbles through the spray-holes and undergoes incomplete combustion. For this reason, the conventional electrolytic polishing process for the removal of the burrs becomes a cause of increased emission of unburned hydrocarbons.
This problem is encountered also in the case of a fuel injection nozzle of the so-called sac-less type in which the inside mouth of each spray-hole is contained in the conical valve seat surface in the nozzle body. In this case the electrolytic polishing process for removing the burrs around the inside mouth of each spray-hole employs a tubular electrode covered with an insulator having a conical end portion which comes into close contact with a major area of the valve seat surface in the nozzle body. However, the valve seat surface is left exposed in a small area where the spray-holes are formed, and an end portion of the tubular electrode protrudes into a space defined by the exposed portion of the valve seat surface and is formed with radial holes to inject the electrolyte liquid toward the respective spray-holes. During the electrolytic polishing process, local dissolution of the valve seat surface occurs in the area exposed to the electrolyte liquid. Therefore, when the needle valve in the completed fuel injection nozzle is in the closing position unintended dead spaces exist between the needle valve and the inner surface of the nozzle body with an adverse effect on the exhaust emission.